Can you reproduce the issue with a sample solution? I think that would be a good starting point.

As mentioned in the other thread, any configuration changes are inherently not thread-safe so they could result in the observed behavior if multiple threads are involved. Similarly, another way the behavior could be observed is if the listener is disposed on
one thread while other threads are attempting to Add LogEntry's (actually,
Add, RetrieveEntries, Flush all check if the repository is set).

Another scenario that might cause the issue is another instance of the application running (perhaps running in the background in an orphaned browser instance?) which causes a failure to open the Isolated Storage.